Composite panel

ABSTRACT

A planar composite panel is constructed from two resin-impregnated fiber face sheet covering and bonded to the two sides of a honeycomb core element and a surrounding border element made of rigid foam board. The two planar faces of the rigid foam board are embossed with a pattern of indentations in the form of interlinked equilateral triangles which are sufficiently deep and close together to provide escape paths for volatiles generated inside the panel during curing of the resin in the face sheets by which the face sheets are bonded to the honeycomb core element and the foam board, to prevent the development of excessive pressure between the face sheets that otherwise would interfere with the bonding.

This is a division of U.S. application Ser. No. 08/235,594 filed on Apr.29, 1994, now U.S. Pat. No. 5,589,016, and entitled "Prescored Foam forPanel Fabrication".

This invention relates to a prescored foam board used in fabrication ofcomposite panels, and more particularly to a method of manufacturingcomposite honeycomb core panels with a foam edge borders withoutmanufacturing defects caused by outgasing from resin pre impregnatedmaterials used for the face sheets of the panel.

BACKGROUND OF THE INVENTION

A modification of airplane interior panels to improve the flammabilityresistance of the panels involves a change of the resin chemistry usedin the resin preimpregnated materials used to form the face sheets ofthe panels. A phenolic resin system provides the desired reduction inflammability, but also creates manufacturing problems because of theincreased volume of volatiles produced during the curing of the resin.This increased volume of volatiles must be accommodated in themanufacturing process to prevent generation of excessive pressure withinthe interior of the panel which prevents bonding of the face sheets ofthe panel to the panel core materials.

The panels are made up from a honeycomb core surrounded by a foamborder, all bonded rigidly together between a face sheet, made of one ormore plies of resin preimpregnated fibrous material ("prepreg"), on eachside of the panel. The elements that make up the panel are bondedtogether by the resin in the prepreg that forms the face sheets and theresin in the honeycomb core. The foam border is provided to give a cleanuniform solid edge surface around the panel which the honeycomb materialwould not provide, allowing for sculpted or contoured edges.

Although the foam border performs its intended function as desired, italso tends to establish a barrier against egress of volatiles generatedor otherwise present during the curing of the resin in thepreimpregnated materials that form the face sheet and/or the honeycombcore during manufacturing. This is especially true when the panels arecured in an oven in which the heating of the panel to cure the resin inthe preimpregnated face sheets tends to heat the panel first around itsperipheral edges and on top, thereby flowing the resin at the peripheraledges first and establishing a seal between the face sheet and the foamborder at a relatively early time in the cycle before all the volatilesin the other parts of the panel have been generated. Then, as theheating progresses and the interior and underside portions of the panelare heated in the oven, the resin cures, generating volatiles in theinterior of the panel that can be trapped therein by the sealestablished earlier around the peripheral edge between the foam borderand the face sheet.

One solution for the need to vent the interior of the panel during thegeneration of volatiles during curing of the resin is to score the foamborder radially to provide gas channels leading from the interior of thepanel to the outside edge of the foam border. This technique usuallyproved successful, but because of the variation in scoring depth andspacing between scoring marks, failures believed to be caused byexcessive interior pressure within the panel during curing stilloccurred at an unacceptable rate. Also, a panel, appearing to haveacceptable quality in the factory, would occasionally develop adelamination once at altitude because of the lower air pressure ataltitude compared with the gas pressure within the sealed panel.Finally, the hand scoring technique was a very time consuming, costly,nonreproducable, and tedious task which the factory workers disliked.Moreover, when the hand scoring was done with a saw, it producedundesirable foam dust and created small cracks and tears in the foamwhich tended to act as stress risers, thereby lowering the strength thatthe foam edge border is expected to provide.

Accordingly, a need has existed for a prescored foam board materialwhich can be used to make foam borders for composite panels, with anoptimum scoring pattern which will provide gas flow channels from theinterior of the panel to the exterior for venting of volatiles withinthe panel during fabrication. This pattern should be equally effectiveregardless of the angle at which the foam pieces are cut when the foamborder is cut from the foam board stock. The scoring pattern should notweaken the foam board excessively. If possible the scoring of the gaschannels in the foam surface should relax and at least partially fill induring the latter stages of manufacturing so that the panel interior canbe sealed against intrusion of moisture or other contaminates during usein the airplane. Finally, the prescored foam should be no more expensiveand preferably less expensive than the manual scoring method.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide an improvedprescored foam board having a pattern of indentations that providesventing of volatiles generated within a composite panel duringfabrication to prevent excessive gas pressure within the panel thatcould interfere with proper bonding of the panel elements duringfabrication. Another object of the invention is to provide an improvedmethod of making a composite panel using prescored foam borders to ventvolatiles in the panel during curing of the resin used to make thepanel. Still another object is to provide a method of making a prescoredfoam board to emboss an indentation pattern into the surface of the foamboard used as a foam border on composite panels, which process preservesthe strength of the board, avoids the generation of dust, and avoidscreation of stress risers in the foam board which could affect itsstrength or service life. Yet another object of the invention is toprovide a composite panel which is sealed around its periphery againstintrusion of contaminates during use but which, during fabrication, hadventing channels for escape of volatiles generated or otherwise presentin the resin used in the materials used to make the composite panel.Still another object of the invention is to provide a method ofprescoring foam board to produce gas channels for venting of volatilesgenerated within the core of the composite panel, which method isreliably effective in venting such volatiles and which also enables thechannels to relax and facilitate sealing of the peripheral border of thepanel against intrusion of contaminates.

These and other objects of the invention are attained in a prescoredfoam used in panel fabrication wherein the surface of the foam board hasthree series of parallel linear indentations, each of which lies at aangle 60° offset from the other two series and which are rolled, stampedsawn or otherwise impressed into the surface of the foam by a laser,ultrasonic knife, patterned roller, flat die, sawing system, or thelike. A roller embossing system is preferred because it which ramps theindentations into the board without creating stress risers. Duringfabrication, the gases generated within the core of the panel are ventedthrough the indentations in the surface of the foam board out to theexterior of the panel, preventing the generation of excessive gaspressure within the panel. A vacuum is drawn inside the panel to drawthe face sheets tight against the honeycomb core for reliable bonding.During heating, the foam material relaxes and springs back to partiallyfill in the indentations, and resin from the pre impregnated materialwhich will form the face sheets flows to completely fill the partialindentations to reseal the peripheral edge of the panel. Any volatilesremaining or generated after the edge of the panel is sealed by theflowing of resin from the face sheet material are accommodated by thevacuum previously drawn in the interior of the panel to pull the facesheets against the honeycomb core material.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its many attendant objects and advantages will becomebetter understood upon reading the description of the preferredembodiment in conjunction with the following drawings, wherein:

FIG. 1 is a perspective view of a panel with a top sheet pealedbackshowing the two plies of the top sheet and revealing the interiorhoneycomb core material and the foam border edge showing the pattern ofindentations formed in the surface of the foam board;

FIG. 2 is a plan view of the structure shown in FIG. 1;

FIG. 3 is an enlarged plan view of a corner section of the honeycombcore element and a portion of the foam border edge shown in FIG. 2;

FIG. 4 is an exploded elevation of the elements that make up a panelshown in FIGS. 1 and 2, shown assembled in a vacuum bag forconsolidation and bonding in an oven;

FIG. 5 is a plan view of a foam board from which individual pieces arecut to form the foam edge frame for a composite panel;

FIG. 6 is a plan view of a typical cell dimension for the pattern ofprestored indentations in the foam sheet shown in FIG. 5.

FIG. 7 is a graph showing the approximate timed sequence of variousparameters related to the process for making the composite panel;

FIG. 8 is a schematic elevation of a roller mechanism for rolling thepattern of indentations shown in FIG. 5 into a rigid foam board;

FIG. 9 is an exploded elevation, similar to FIG. 4, showing analternative embodiment using electric heaters instead of an oven; and

FIG. 10 is a schematic plan view of an electric heater for use in theassembly shown in FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings, wherein like reference characters designateidentical or corresponding parts, and more particularly to FIG. 1thereof, a panel 30 is shown having a portion of a top sheet 31 pealedback to illustrate the interior construction of the panel. The panel 30is made of interior components covered on both sides by face sheets 31.The interior components include a planar core element such as ahoneycomb core element 32, and an edge border 34 of a closed cell foamsuch as polyurethane foam disposed around the honeycomb core element 32.The edge border element 34 can be made of a plurality of separate pieces35 jointed and/or glued together at their junctions 36, as illustratedin FIG. 2, or can be a single piece cut from a large piece of foamboard. Preferably, the foam edge border 34 is made up of a plurality ofseparate pieces 35 since the advantages of a single piece constructionare outweighed by cost of the waste that would be caused by making theedge border 34 out of one piece without junctions 36. The separatepieces preferably include the corner instead of terminating at thecorner, and are jointed together with a dovetail joint for strength andsecurity against shifting apart during assembly and before bondingbetween the face sheets 31. The honeycomb core element 32 is made ofphenolic resin, epoxy or other resin system preimpregnated Nomex paperformed into hexagonal cells 37 oriented with the cell axis perpendicularto the plane of the panel, as illustrated in FIG. 3. Alternatively, thehoneycomb could be made with metallic foil or thermoplastic film.

Face sheets 31 are bonded on each side of the core element 32 and thesurrounding edge border 34. The face sheets 31 are each made of one ormore, preferably two, plies 38 of resin impregnated woven, braided, orunwoven fibrous material such as fiberglass fabric, Kevlar, graphite orthe like material, commonly referred to as "prepreg", which are bondedtogether and bonded to the interior components with the impregnatingresin in the prepreg. The resin selected for the use in the prepreg isphenolic resin, chosen for its physical characteristics, including flameretardance and resistance to flammability, light weight and low emissionof noxious fumes in the presence of flame, as well as its mechanicalproperties of strength and stiffness. Prepregs using other polymersystems also suffer similar panel core pressure manufacturing problemsand will benefit from this invention.

Although the phenolic resin has desirable physical properties, its usein the manufacture of the panel 30 has created manufacturing problemsbecause of the large volume of volatiles which it generates during thecuring process. In manufacturing the panel 30, the elements 40 whichmake up the final panel, namely the interior elements consisting of thehoneycomb core element 32 and the surrounding foam edge border 34, andthe two prepreg sheets 38 on each side of the interior elements, areassembled as illustrated in FIG. 4, between two sheets 42 and 42' of arelease film, which assembly in turn is sandwiched between two sheets 44and 44' of breather material. The entire assembly is then covered with avacuum bag 46 of a heat resistant gas impervious material such as nylonfor phenolic resin prepregs, PVC for epoxy prepregs or a silicon blanketsealed to a top surface 50 of a table 52 with a conventional sealingmaterial 48. A series of vacuum ports 54 communicates through a strip 45of breather material, which connects all the ports 54, with the surfaceof the table 52 by way of a gas pipe 56 and is connected by a coupling58 to a source of vacuum such as a vacuum pump 60 through a vacuum line62.

In operation, the assembly shown in FIG. 4 is sealed in the vacuum bag46 to the top surface 50 of the table 52 by the seal material 48. Thevacuum pump 60 is connected to the coupling 58 by way of the vacuum line62 and air is evacuated from within the sealed enclosure within thevacuum bag 46. The withdrawal of air from within the vacuum bag 46 hasthe dual advantage of causing atmospheric air pressure exerted on thetop surface of the bag 46 to press downward on the assembled elementswithin the bag, exerting a downward pressure on the top plies 38pressing them against the core elements 32 and 34 and pressing thoseagainst the bottom plies 38'. This pressure will be effective inpromoting bonding of the elements 40 together when the assembly isheated in an oven to flow the resin in the prepreg plies 38 and 38'. Thesecond advantage of the vacuum in the bag 46 is that the air within thecells of the honeycomb element 32 is evacuated, establishing a lowpressure reservoir which can accommodate the residual out gassing fromthe phenolic resin which may later be trapped by sealing of the panelelements around the peripheral edge, as described below.

After the vacuum has been established within the vacuum bag 46, thevacuum line is disconnected from the coupling 58, and the table 52 withthe vacuum bag 46 and its assembled elements is transported into an ovenalong with numerous similar tables and assembled vacuum bags. The vacuumwithin the vacuum bag 46 is maintained by a one way valve 64 in thevacuum line 56, preventing intrusion of air back into the vacuum bag 46.In the oven, the coupling 58 is reconnected to another vacuum line 62connected to the vacuum pump 60 to withdraw volatile gases generated orotherwise present during curing of the phenolic resin in the oven, andto maintain the consolidating pressure exerted by the atmosphere throughthe bag 46 on the plies 38 and 38'.

In the oven, power to resistance heating elements 66 is applied and airis circulated through a recirculating air supply line 68 to be heated bythe elements 66 and circulate within the oven around the vacuum bag 46to heat the bag and its contents and also heat the table 52. As thetemperature rises, the resin preimpregnated in the prepreg sheets 38 and38' begins to become less viscous and flows, wetting the prepreg sheets,the facing edges of the honeycomb core element 32, and the foam edgeborder 34. During this period, the phenolic resin in the prepreg sheets38 produces significant volumes of volatiles, primarily water vapor,formaldehyde and free phenol which must have an escape path from theinterior of what will become the panel when it is completely bondedtogether. Unless an escape path is provided for these volatiles, theywill generate sufficient pressure within the core of the panel betweenthe face sheets 31 and 31' to interfere with complete bonding of thesheets 38 and 38' to the honeycomb core element 32 and possibly the foamedge border pieces 34. For that purpose, a pattern of indentations shownin FIG. 5 is provided on the surface of the foam edge border pieces.

A foam board 80 of rigid polyurethane closed cell foam is shown in FIG.5 having a pattern of surface indentations 82 embossed into the surfaceof the foam board 80. The pattern is in the form of three series ofparallel lines all angularity offset from each other by 60°. Arepresentative set of three lines from each series is illustrated inFIG. 5 as 82A, 82B and 82C. As illustrated, the linear indentations ofseries 82A in the board 80 extends parallel to the longitudinal axis ofthe board, and the two series 82B and 82C extend 60° from the series 82Ato the right and to the left respectively. As illustrated, the linearindentations 82A, 82B and 82C all intersect at single points throughoutthe pattern, but the pattern could have been arranged with only twolines intersecting at any one point.

As shown in FIG. 6, the pattern of linear indentations 82 in the board80 forms a series of contiguous equilateral triangles 84. For the sizepanel being constructed, a pattern in which the linear indentations arespaced about 13/4 inches apart produces an equilateral triangle havingsides all equal to about 2 inches in length. This was found to produce asatisfactory series of venting channels for the volatile vaporsgenerated inside the panel during curing of the phenolic resin. Thewidth of the indentations is about 1/16th inch and the depth is about1/16" to 3/16" deep. The dimensions could be adjusted to match thevacuum requirements of differing processes, constructions, or resinsystems.

The indentation pattern shown in FIG. 5 can be cut into the foam boardusing a saw with a depth gauge and automatic indexing of the board aftereach saw cut. Likewise, it could be formed by a series of die strokesusing a flat or curved die having the pattern shown stamped into theboard. These techniques produce adequate patterns of linearindentations, however they also produce foam dust in the case of sawingand small cracks or tears in the foam board in the case of sawing andstamping. In most cases, these problems will not significantly affectthe manufacture or use of the panel, so they may be disregarded.However, in situations where they may be considered undesirable, apreferable technique may be used, as illustrated in FIG. 8, using twoembossing rollers 90 and 92 having the die pattern shown in FIG. 8 setin their exterior surface. The two embossing rollers 90 and 92 arespaced longitudinally in the direction of motion of the board 80indicated by the arrow 94. Preferably, each roller 90 and 92 has a solididle roller 96 and 98, respectively, positioned on the opposite side ofthe board 80 to provide a reaction force to the force exerted by theembossing rollers 90 and 92. As illustrated, the pattern embossed intothe top surface of the board 80 by the roller 90 is offset from the samepattern embossed in the lower surface of the board 80 by the roller 92so that none of the embossed linear indentations coincide which couldform a weak hinge point in the foam board 80.

Besides the advantages of speed and cleanliness provided by the rollingtechnique for embossing the pattern of indentations 82 into the foamboard 80, the rolling technique also avoids the generation of smallcracks and tears in the foam material which occur with the stamping andsawing technique and serve as stress risers which can form the start offatigue cracks in the material. The avoidance of the stress risers isbelieved to occur because of the gradual ramping of the indentationsinto the material at a small ramp angle Φ. This angle Φ, about 10°-35°,preferably about 15°, allows the die surfaces in the rollers 90 and 92to gradually displace the material in the surface of the foam board 80and do so in localized areas of the foam board surface. This allowsother material immediately upstream of the die contact area to prestrainand distribute the stress laterally, so when that area is strained bythe rolling die it does not tend to result in the small tears and cracksthat are observed with the use of a flat die.

Turning now to FIG. 7, a graph is shown having time as the abscissa andvarious parameters of the process such as temperature, rate of volatilegeneration, core pressure, and resin viscosity as separate ordinates.The graph of FIG. 7 shows the change in these parameters as the cycleprogresses from beginning to end. At time zero, all of the tables 52have been loaded into the oven, the vacuum lines 62 connected to all ofthe couplings 58, oven door closed and the heater elements 66 energized.The temperature begins to climb and reaches a temperature of about 200°F. in about 12 minutes. At about this temperature, the resin reachesgelling temperature, its viscosity falls, and the resin flows, wettingthe surfaces in contact with the prepreg materials. Also at thistemperature, the evolution rate of volatiles increases rapidly.Volatiles, primarily water vapor, are generated rapidly and continue tobe generated for the next 5 or so minutes. At this phase in the cycle,the cross linking of polymers in the phenolic resin matrix accelerates.As the cross linking proceeds the resin viscosity increases and itbecomes more difficult for volatiles generated in the core of the panelto escape through the embossed indentations in the surface of the foamedge border 34. Eventually, the resin will become viscous enough toprevent further escape of volatiles generated within the core of thepanel and the vacuum established by the vacuum pump 60 in the core ofthe panel will begin absorbing these volatiles. The residual generationof volatiles in the core after the prepreg face sheets 31 bond to thefoam edge border to seal the peripheral edge of the panel results in adecrease of the vacuum, or increase in pressure, in the core, as shownin the graph 100C.

Since the natural heat flow into an article lying on the table 52 in theoven is from the top and sides of the heated article inward, the topsurface and peripheral edges of the panel elements shown in FIG. 2 willtend to be heated first. The center of the panel on the table side tendsto reach gelling temperature last. This situation could potentiallycause the peripheral edge of the panel elements to be heated to gellingtemperature and begin cross linking before the evolution of volatiles inthe lower central portion of the panel has commenced. In such asituation, especially in the fabrication of very large panels, thiscould potentially result in premature sealing of the peripheral edge ofthe panels before evolution of the volatiles from the ungelled resin,and the development of excessive pressure in the core of the panel couldoccur. In such a situation, a refinement of the heat application can beused to tailor the sequence of gelling in zones of the panel to preventpremature sealing of the panel periphery and enhance the efficacy of thescoring pattern in the foam edge boarder.

To alter the pattern of heating in the oven to heat from the centeroutward rather than from the peripheral edge inward, as shown in FIG. 4,a pair of heat ducts 110 and 112 is provided above and below each table52 having suitable gas respective nozzle 114 and 116 disposed over thecenter of the area in which the panel will be positioned. The nozzles114 and 116 will direct the heated air in the convection oven againstthe top and bottom center of the vacuum bag 46 and the table 52respectively causing the center of the bag assembly and table to heat upfirst. This arrangement will cause the gelling of the resin in theprepreg sheets 38 and 38' to occur first in the center, thereby leavingopen a path for egress of the ensuing volatiles before the peripheraledge of the part becomes sealed when the resin at the peripheral of thepart gells.

The same zone controlled heating illustrated by the use of the ducts 110and 112 in FIG. 4 is also achieved in a second embodiment of theinvention as shown in FIG. 9, wherein a pair of electrical heaters a topheater element 120, and a lower heater element 120, shown in FIG. 9, arerespectively disposed above and below the same elements 40 through 44shown in FIG. 4. An insulator 122 is placed over the top of the topheater element 120 and the entire arrangement is sealed in a vacuum bag46 to a table 124 by a sticky sealant 48.

The heater 120 is a flexible blanket heater having a series of heaterelements 126, 128, 130 etc. disposed concentrically around the centerheater element 126, as shown in FIG. 1. Lead wires connect from theperipheral edge of the blanket heater 120 to each of the heater elementsand each element is connected to a common ground 132. The lower heaterelement 120' can be set into the table 124 with internal leads 134 orcan be a separate heater element lying on the surface of the table 124.

In operation, the elements 40 through 44 are assembled on the heaterelement 120' and covered with the heater element 120 and the insulationblanket 122. The vacuum bag 46 is placed over the assembly and sealed tothe table 124 with a sealant 48. The vacuum pipe 56 is connected by itscoupling 58 to a vacuum line leading to a source of vacuum 60, and avacuum is drawn inside the vacuum bag 46. After the vacuum isestablished and the atmospheric pressure is exerting a uniform downwardforce on the top of the bag 46 to compress all of the elements 40 thatwill form the panel, the center element 126 in the top and bottomheaters 120 and 120' is energized to begin heating the center of theassembly. After a suitable interval of about 5 minutes, the nextconcentric radial heater 128 adjacent to the inner heater 126 isenergized to begin heating the next radial zone outside the center zone126. The heating of the panel elements progresses radially outwardly inthis manner. As the gelling and curing of the resin progresses from thecenter outward, the volatiles generated during the gelling and curingprocess can escape radially outward through the linear indentations 82in the foam edge board pieces 34 since the resin at the peripheral edgesof the panel has not yet begun to gell and fill the indentations 82. Theelements 128, 130 et cetera are energized in a sequence that provides asuitable time period for the generation of the volatiles to progressthrough its normal maximum generation rate and begin tapering off beforethe next element raises the temperature to cause the generation ofvolatiles in the next radially outward zone to peak, so that all of thevolatiles can be drawn out from the interior of the panel thorough thevacuum pipe system 56 before the peripheral edge around the foam edgeborder 34 is heated and sealed.

Obviously, numerous modifications and variations of the describedpreferred embodiments will occur to those skilled in the art in view ofthis disclosure.

Accordingly, it is expressly to be understood that these modificationsand variations, and the equivalents thereof, may be practiced whileremaining within the spirit and scope of the invention as defined by thefollowing claim, wherein we claim:
 1. A planar composite panel havingtwo opposed planar faces, comprising:two resin-impregnated fiber facesheets covering and bonded to two opposed sides of a core element; and aborder element made of foam board surrounding said core element, saidborder element including a plurality of foam strips each of said foamstrips having a pattern of indentations within at least one side of saidfoam board, said pattern of indentations having three series ofindentations within said foam board, each series being angularly offsetfrom each other two series by 60°; said resin being of the type thatgenerates relatively large volumes of volatile vapors during curing inthe course of said bonding, said volatile vapors having been vented fromsaid core element between said face sheets though said series ofindentations within said foam board.
 2. A composite panel as defined inclaim 1, wherein:said curing of said resin to bond said face sheets tosaid core element and said border element is by heating a stack-upincluding said face sheets positioned over and under said core elementand said border element, said resin when heated to a temperature atwhich it becomes fluid, flows to wet said core element and said borderelement, and then becomes viscous and bonds said face sheets to saidcore element and said border element as it polymerizes.
 3. A compositepanel as defined in claim 2, wherein:said pattern of indentations have apredetermined depth within the side of said foam board such that saidfoam board partially relaxes to partially restore said border element toits preembossed, unindented condition during heating to cure said resinand bond said face sheets to said core element and said border element;and during said heating, said resin in said face sheets flows into andfills remaining partial indentations and bonds said face sheets to saidborder element to seal said interior of said panel within said facesheets and said border element.
 4. The composite panel according toclaim 3, wherein spacing between the indentations in each of said seriesof indentations is approximately 13/4 inches and wherein thepredetermined depth of each of said three series of indentations isapproximately 1/16 to 3/16 inches.
 5. A composite panel as defined inclaim 2, wherein:said heating of said stack-up is from a center zone ofsaid stack-up first, allowing said volatile vapours to vent through saidindentations, and thereafter an intermediate zone radially outside saidcenter zone is heated and said volatile vapours are vented out throughsaid indentations, and then an outside peripheral zone is heated to bondsaid face sheets to said border element to seal said interior of saidpanel within said face sheets and said border element.